Quality of service (QoS) in LTE and NR

Quality of service (QoS) refer to the measurement of the overall performance of a service experienced by the users of the network. To quantitatively measure the QoS packet loss, bit rate, throughput, transmission delay, availability, jitter, etc. related aspects of service are considered. As per ITU definition, QoS comprises requirements on all the aspects of a connection, such as service response time, loss, signal-to-noise ratio, crosstalk, echo, interrupts, frequency response, loudness levels.

Following figure show the performance of channel Bandwidth (BW) with and without QoS.

In 4G, LTE QoS is enforced at the EPS bearer level. In 5G, QoS is enforced at the QoS flow level. 4G LTE uses EPS bearers each assigned an EPS bearer ID. In 4G/LTE, QoS is applied at the level of Evolved Packet Service (EPS) bearer. There's a one-to-one mapping, which really means that for an EPS bearer there's a corresponding EPS Radio Access Bearer (RAB), an S1 bearer and a Radio Bearer (RB).

In 5G NR, QoS is enforced at the QoS flow level. Each QoS flow packets are classified and marked using QoS Flow Identifier (QFI). The 5G QoS flows are mapped in the Access Network to DRBs (Data Radio Bearers) unlike in 4G where mapping is one to one between EPC and Radio Bearers. As with 4G LTE both non-GBR flows and GBR flows are supported in 5G, along with a new delay-critical GBR. 5G also introduces a new concept - Reflective QoS. 5G QoS architecture supports following QoS flow types.

? GBR QoS flow which requires guaranteed flow bit rate

? Non-GBR QoS flow which does not require guaranteed flow bit rate

? Delay Critical QoS flow, For Mission Critical guaranteed flow bit rate

The QoS flow is the lowest level granularity within the 5G system and is where policy and charging are enforced. One or more Service Data Flows (SDFs) can be transported in the same QoS flow, if they share the same policy and charging rules (similar to an EPS bearer in 4G LTE). All traffic within the same QoS flow receives the same treatment.

 There are several standardized 5QI values. The following table, from 3GPP TS 23.501, provides the mapping from 5QI to QoS characteristics.

5G NR QoS Parameters: 3GPP specification 23.501 defined the following 5G QoS Parameters as part of QoS Implementation.

? 5G QoS Identifier (5QI): An identifier for QoS characteristics that influence scheduling weights, admission thresholds, queue management thresholds, link layer protocol configuration, etc.

? Allocation and Retention Priority (ARP): Information about priority level, pre-emption capability (can pre-empt resources assigned to other QoS flows) and the pre-emption vulnerability (can be pre-empted by other QoS flows).

? Reflective QoS Attribute (RQA): Optional parameter. Certain traffic on this flow may use reflective QoS.

? Guaranteed Flow Bit Rate (GFBR): Measured over the Averaging Time Window. Recommended to be the lowest bitrate at which the service will survive.

? Maximum Flow Bit Rate (MFBR): Limits bitrate to the highest expected by this QoS flow.

? Aggregate Maximum Bit Rate (AMBR): Session-AMBR is per PDU session across all its QoS flows. UE-AMBR is for each UE.

? QoS Notification Control (QNC): Configures NG-RAN to notify SMF if GFBR can't be met. Useful if application can adapt to changing conditions. If alternative QoS profiles are configured, NG-RAN indicates if one of these matches currently fulfilled performance metrics.

? Maximum Packet Loss Rate: In Release 16, this is limited to voice media.

 Summary

5G builds upon and enhances QoS capabilities compared to 4G, and the terminology has changed. The standardized QoS options are extended as indicated in the 5QI to QoS characteristics mapping table, and additional QoS parameters are defined.

Thanks and see you soon in the coming article,

Hassan QADI